To improve the thermal efficiency of gasoline internal combustion engines, dilute combustion, using either air or EGR, is known to give enhanced thermal efficiency and lower NOx emissions. There is, however, a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the dilution limit include: 1) improving ignitability of the mixture by enhancing ignition and fuel preparation; 2) increasing the flame speed by introducing charge motion and turbulence; and 3) operating the engine under controlled auto-ignition combustion.
The controlled auto-ignition process is sometimes called a Homogeneous Charge Compression Ignition (HCCI). In this process, a mixture of combusted gases, air, and fuel is created and auto-ignition is initiated simultaneously from many ignition sites within the mixture during compression, resulting in very stable power output and high thermal efficiency. The combustion is highly diluted and uniformly distributed throughout the charge. The burned gas temperature and hence NOx emissions are substantially lower than that of traditional spark ignition engines based on propagating flame front and diesel engines based on an attached diffusion flame. In both spark ignition and diesel engines, the burned gas temperature is highly heterogeneous within the mixture with very high local temperature creating high NOx emissions.
Engines operating under controlled auto-ignition combustion have been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burned gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture. In four-stroke engines with traditional valve means, the residual content is low, and controlled auto-ignition at part load is difficult to achieve. Known methods to induce controlled auto-ignition at low and part loads include: 1) intake air heating; 2) variable compression ratio; and 3) blending gasoline with ignition promoters to create a more easily ignitable mixture than gasoline. In all the above methods, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow.
Engines operating under controlled auto-ignition combustion have been demonstrated in four-stroke gasoline engines using variable valve actuation with unconventional valve means. The following is a description of one such unconventional valve strategy. With this valve strategy, a high proportion of residual combustion products from previous combustion cycles is retained to provide the necessary condition for auto-ignition in a highly diluted mixture. The range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is greatly expanded using a conventional compression ratio.
A method of operating a four-stroke internal combustion engine has been disclosed in which combustion is achieved at least partially by an auto-ignition process. Flow of premixed fuel/air charge and combusted gases is regulated by hydraulically controlled valve means in order to generate conditions in the combustion chamber suitable for auto-ignition operation. The valve means used includes an intake valve controlling flow of premixed fuel/air mixture into the combustion chamber from an inlet passage and an exhaust valve controlling flow of exhaust combusted gases from the combustion chamber to an exhaust passage. The exhaust valve is opened for two separate periods during the same four-stroke cycle. The exhaust valve is opened for a first period to allow combusted gases to be expelled from the combustion chamber and for a second period to allow combusted gases previously exhausted from the combustion chamber to be drawn back into the combustion chamber. The double opening of the exhaust valve during each four-stroke cycle, creates the necessary condition for auto-ignition in the combustion chamber. This is generally referred to as an exhaust re-breathing valve strategy.
A method of operating a four-stroke internal combustion engine has also been disclosed in which combustion is achieved at least partially by an auto-ignition process. Flow of air and combusted gases are regulated by hydraulically controlled valve means as detailed above. The fuel, however, is delivered by a gasoline injector directly into the combustion chamber. The gasoline injector is said to inject fuel either during the intake stroke or the subsequent compression stroke during a single engine cycle.
In general, HCCI engine operation is limited by combustion stability at low engine load and by in-cylinder pressure rise or amplitude of pressure oscillation at a mid load limit. Too large a pressure rise or amplitude of pressure oscillation results in combustion generated noise called ringing. It has been found experimentally that both internally and externally recirculated burned gas is effective in controlling the combustion rate and hence the pressure rise. The present invention describes a method for regulating the thermal energy of the internally recirculated burned gas or internal residual as an alternative to external EGR for HCCI engine combustion control in the mid load range.